Gas turbine engines, such as those used on jet aircraft, generally comprise an air intake port, a fan mounted on a hub near the air intake port and surrounded by a fan case, a low pressure compressor (LPC) section, an intermediate section aft of the LPC section, a high pressure compressor (HPC) section, a combustion chamber or combustor, high and low pressure turbines that provide rotational power to the compressor blades and fan respectively, and an exhaust outlet. The fan and LPC section may be operably connected to the low pressure turbine by an inner drive shaft which rotates about an engine center axis. A cone-like spinner may be mounted over the hub forward the fan blades to help guide air flow.
Some sections of the engine include airfoil assemblies comprising airfoils (typically blades or vanes) mounted at one or both ends to an annular endwall. Included among these sections is the fan section in which fan blades drive air flow into the engine core.
Weight reduction in gas turbine engines used for aircraft results in fuel savings. One known means for reducing the weight of a gas turbine engine is to include hollow cavities in some of the components that do not need to be solid metal to meet structural requirements. One such component is the fan blade.
Some fan blades comprise a metallic body made of titanium or aluminum or other metallic materials with an opening located on the non-flow path, convex side of the fan blade, also known as the suction side of the fan blade, wherein the opening communicates with recesses or cavities. The opposite side of the fan blade is the concave or pressure side. The opening is covered by a composite cover, typically made from fiber and resin plies. This disclosure applies to fan blades where the fan blade body preferably is made of a denser material than the cover.
The cover has a cover leading edge and a cover trailing edge which generally correspond to the fan blade leading edge and the fan blade trailing edge. The cover may have a constant thickness. However, the thickness of the fan blade varies because of its airfoil shape. Consequently, the thickness of the fan blade body varies, and is smallest near the leading edge and trailing edge, especially near the blade tip. This configuration can result in the fan blade body under the leading edge and the trailing edge of the cover being too thin to provide sufficient strength against liberation of the edge of the fan blade body.
This problem can be addressed by moving the cover leading and trailing edges of the relatively lighter (less dense) cover inward, but the resulting fan blade will have greater mass along its leading and trailing edges near the blade tip, which could lower the torsional stiffness of the blade. If the lower torsional stiffness leads to a low order natural frequency of the blade that is in the engine operating range, that circumstance can cause excessive torsional vibration during operation which can be deleterious to fan blade performance and fan blade life.
The present disclosure addresses these problems and others.